In an internal combustion engine feeding aqueous urea solution in an engine exhaust passage to reduce the NOx in the exhaust gas, there is provided an aqueous urea solution tank for storing aqueous urea solution. Ammonia gas vaporized from the aqueous urea solution fills this aqueous urea solution tank, accordingly, removing the lid of the refilling inlet of the aqueous urea solution tank to refill the aqueous urea solution will release ammonia gas into the outside air.
Therefore, to prevent such release of the ammonia gas into the outside air, there is known an aqueous urea solution tank provided with an electric fan for forcibly exhausting the ammonia gas generated in the aqueous urea solution tank to the top of the aqueous urea solution tank, an adsorption apparatus for temporarily adsorbing the ammonia gas exhausted by the electric fan, and an oxidation catalyst for oxidizing the ammonia desorbed from the adsorption apparatus (for example, refer to Japanese Patent Publication (A) No. 2005-105915).
However, in this aqueous urea solution tank, there is the problem of the generated ammonia gas being needlessly consumed.